Use a different domain for SSL

Just to keep things nice & clear, I’ve posted this as a new answer. Let’s reset the playing field & follow the below instructions as if it were a shiny new install (ignore all code & suggestions in previous answers).

In your wp-config.php

define( 'WP_SITEURL', 'http://www.realdomain.com/blog' );
define( 'SSL_DOMAIN_ALIAS', 'realdomain.maindomain.net' );

define( 'FORCE_SSL_LOGIN', true );
define( 'FORCE_SSL_ADMIN', true );

And in wp-content/mu-plugins/ssl-domain-alias.php

<?php

/**
 * Plugin Name: SSL Domain Alias
 * Plugin URI: http://wordpress.stackexchange.com/questions/38902
 * Description: Use a different domain for serving your website over SSL, set with <code>SSL_DOMAIN_ALIAS</code> in your <code>wp-config.php</code>.
 * Author: TheDeadMedic
 * Author URI: http://wordpress.stackexchange.com/users/1685/thedeadmedic
 *
 * @package SSL_Domain_Alias
 */

/**
 * Swap out the current site domain with {@see SSL_DOMAIN_ALIAS} if the
 * protocol is HTTPS.
 *
 * This function is not bulletproof, and expects both {@see WP_SITEURL} and
 * {@see SSL_DOMAIN_ALIAS} to be defined.
 *
 * @todo The replacement is a simple string replacement (for speed). If the
 * domain name is matching other parts of the URL other than the host, we'll
 * need to switch to a more rigid regex.
 *
 * @param string $url
 * @return string
 */
function _use_ssl_domain_alias_for_https( $url )
{
    static $domain;
    if ( ! isset( $domain ) )
        $domain = defined( 'WP_SITEURL' ) && defined( 'SSL_DOMAIN_ALIAS' ) ? parse_url( WP_SITEURL, PHP_URL_HOST ) : false;

    if ( $domain && strpos( $url, 'https' ) === 0 )
        $url = str_replace( $domain, SSL_DOMAIN_ALIAS, $url );

    return $url;
}
add_filter( 'plugins_url', '_use_ssl_domain_alias_for_https', 1 );
add_filter( 'content_url', '_use_ssl_domain_alias_for_https', 1 );
add_filter( 'site_url', '_use_ssl_domain_alias_for_https', 1 );

?>

I’ve suggested using a Must-Use plugin (mu-plugins), since these are autoloaded without having to be activated first.

If you’d rather it be a standard plugin, you’ll need to add the FORCE_SSL_* constants after activation.